1. Field of the Invention
The invention generally relates to high power semiconductor devices, and more particularly to field termination structures used for enhancing voltage blocking abilities of semiconductor devices.
2. Description of the Related Art
Several field termination structures and techniques are presently utilized in the industry. These include field plates, field rings, beveling, and surface implant termination or junction termination extension (JTE) and they all tend to increase the voltage blocking ability of semiconductor devices. As known to those skilled in the art, field plates are metal contacts that do not conduct but instead alter the surface potential to reduce the curvature of electrostatic fields at the surface of the semiconductor device. Usually, the field plates are isolated from the semiconductor material by an insulating oxide layer disposed in between the field plates and the semiconductor material. Moreover, field rings tend to extend the depletion region boundary along the surface of the semiconductor device and are typically embodied as implanted impurities in the semiconductor surface.
Beveling the edges of a vertical power semiconductor device has been shown to promote structural breakdown to occur within the bulk material of the semiconductor device rather than at the surface by reshaping the device so that higher fields are within the device. Additionally, surface implant termination or JTE usually achieves a less abrupt high to low dopant concentration transition with a gradual decrease in the dopant concentration between the high doped region and the drift region of the semiconductor device. In view of the above techniques, the industry has generally concluded that the JTE is the best field termination to use.
High power semiconductor devices are typically made from silicon and are used in applications of up to several kilovolts. However, applications using high power semiconductor devices may have been limited because of the lack of a p-n junction in silicon bipolar power devices when operating above 150° C. as is easily reached when handling high powers. Also, for some silicon devices to block very high voltages, either the drift region had to be very thick, or several devices had to be connected in series. Both cases are inferior because thicker drift regions lead to slow switching, and having several devices in series may cause reliability problems unless the devices are identical and display the same characteristics over time. Nonetheless, the industry continues to strive to develop high power semiconductor devices which achieve high voltage blocking with low on-state voltage over a large current range with fast switching.